Acrylic pressure-sensitive adhesives and rubber-based pressure-sensitive adhesives are widely known as pressure-sensitive adhesives. Especially recently, acrylic pressure-sensitive adhesives tend to be used in extensive applications ranging from strong-tack pressure-sensitive adhesives having high adhesive force to slight-tack pressure-sensitive adhesives having low adhesive force for use in, e.g., protective films for motor vehicles and electronic materials. However, acrylic pressure-sensitive adhesives have posed problems concerning odor and skin irritation when acrylic monomers remain in the pressure-sensitive adhesives. Furthermore, a removable acrylic pressure-sensitive adhesive is, with the lapse of time after application to an adherend, apt to increase in adhesive force or come to have enhanced migration property, thereby causing an adhesive deposit on the adherend. There have hence been cases where removability becomes insufficient. In addition, a substrate coated with an acrylic pressure-sensitive adhesive has poor followability to adherends. Because of this, when the adherend shape is changed, there are cases where the coated substrate cannot follow the adherend change and peels off or breaks. In rubber-based pressure-sensitive adhesives, the addition of a low-molecular weight plasticizer is indispensable for handleability and for controlling pressure-sensitive adhesiveness, and there has been a problem that the low-molecular weight plasticizer migrates to the surface with the lapse of a prolonged time period to cause a considerable decrease in performance.
In view of the problem, urethane-resin-based pressure-sensitive adhesives obtained by reacting a urethane resin as a main ingredient with a polyisocyanate compound as a crosslinker have been proposed. Examples of the urethane resin serving as a main ingredient which have been proposed include one obtained by reacting a polyol with a polyisocyanate to obtain an isocyanate-terminated urethane prepolymer (isocyanate-group-terminated prepolymer) and reacting this prepolymer with a chain extender having three or more functional groups reactive with an isocyanate group (see patent document 1). The functional groups of the chain extender in patent document 1 can be primary and secondary amino groups and primary, secondary and tertiary hydroxyl groups. Of these functional groups, the primary and secondary amino groups and the primary hydroxyl group are used for the reaction with the isocyanate-terminated urethane prepolymer and react with molecular ends of the isocyanate-terminated prepolymer to connect molecules of the urethane prepolymer to each other. The remaining secondary and/or tertiary hydroxyl group remains in the urethane resin and is used for reaction with the polyisocyanate compound as a crosslinker. With this urethane resin, the adhesive force and cohesive force of a pressure-sensitive adhesive can be controlled in accordance with the reaction amount of the residual secondary hydroxyl groups or tertiary hydroxyl groups with the crosslinker.
Patent Document 1: JP-A-2003-12751